In recent years, a radio communication system capable of achieving high-speed communications by use of Orthogonal Frequency Division Multiplexing (OFDM) has been put to practical use. For example, Mobile WiMax defined in IEEE 802.16e is known as such a radio communication system.
In Mobile WiMAX, a radio communication terminal receives channel allocation information (DL-MAP and UL-MAP) broadcasted from a radio base station by a non-directional beam. The radio communication terminal acquires information on an uplink radio communication channel, specifically, a ranging sub-channel, on the basis of the received channel allocation information, and then transmits a ranging code (ranging information) by use of the ranging sub-channel.
Upon reception of the ranging code from the radio communication terminal, the radio base station allocates a frequency bandwidth necessary for communications to the radio communication terminal. Meanwhile, when the radio communication terminal cannot receive the channel allocation information transmitted through a map region in a downlink frame, for example, when the radio communication terminal cannot receive the channel allocation information because of being far from the radio base station, the radio communication terminal can acquire information on an uplink ranging sub-channel by referring to a specific region in the downlink frame, specifically, an AAS-DLFP (adaptive antenna system-downlink frame prefix) (refer to Non Patent Document 1). The AAS-DLFP (AAS pointer) is transmitted from the radio base station in a predetermined direction by a directional beam by use of an array antenna.    Non-Patent Document 1: “IEEE 802.16e-2005,” [online], February 2006, [retrieved on Jan. 24, 2007], Internet URL: http://standards.ieee.org/getieee802/download/802.16e-2005.pd f